Imported Objects and UV Maps
When you import an object from another .blend file with Link or Append from the File menu, it comes with all linked datablocks, so UV maps, materials, textures, and various others will be also imported.

When you import from a different file format with File > Import, what you get depends on what the format stores. For example, "Wavefront (.obj)" can contain mesh, UV map, and texture, but not armature. When you import an SVG file, you will get several curve objects.

For an example of using multiple .blend files in a project, look at the files from Big Buck Bunny.